This invention relates to a surface friction tester and, more particularly, to a portable friction sled which can be used to determine the coefficient of friction of a surface; thereby, enabling accurate calculation of the vehicle velocity at the time of brake locking from the skid marks left by vehicle sliding on that surface.
Traditionally and dutifully the efficient law enforcement officer will measure the length of skid marks which inevitably mark the auto accident scene. Giving attention to procedures for estimating speed based upon the application of the laws of physics (from the point of brake locking) there are a recognized group of scientific procedures to assist the investigator. A very "workable" formula has been used. It is: V.sup.2 =30 f s, where v=velocity (mph) of the auto at the locking of the brakes; f=coefficient of friction of the road surface; and s=feet of measured skid marks.
At present, the investigating officer determines the coefficient of friction (f) by laying down a set of skid marks with his cruiser, applying the brakes at a known velocity. By measuring the skidmarks and utilizing the above given formula, he then calculates the coefficient of friction figure (f) to be used in determining (using the same formula) the speed of the wrecked vehicle at the time of brake lock. However, this equation, through widely used by traffic investigators, is not adequate when the skid conditions occur concurrently and/or consecutively on a variety of road surfaces.
In that instance, it is necessary to use a more complex formula utilizing the various coefficients of friction of the different surfaces over which the vehicle has skid. The problem is that these measurements obviously cannot be obtained by laying down a skid mark with the cruiser since it is difficult and dangerous to try to duplicate such a slide. In addition, there is presently no inexpensive, portable surface friction tester which can be easily used at the accident scene for each of these different surfaces. Rather, the types of surface friction testers known are not totally suitable for that surpose. Instead, they are designed principally for measuring the skid resistance of a highway and not for auto accident investigators, including skid conditions on several different types of surfaces.
As mentioned in Lynch U.S. Pat. No. 3,367,170, there are three general types of apparatus for testing the frictional characteristics of a road surface. First there are devices with a wheel fitted with a brake drum. When the wheel is pulled over a surface, a force is applied to the brake drum, and when this force is great enough to stop the rolling of the wheel, a measure of the frictional characteristic of the road surface is obtained. Most of the ones of this type are automobile or trailer mounted. Patents illustrating this type device are U.S. Pat. Nos. 3,893,330; 3,538,742; 3,431,776 and 2,700,297.
A second type is the British Portable Tester, which is described by Lynch as being a stationary device composed of a stand supporting a pendulum having a rubber shoe on the pendulum. The pendulum is lifted manually and then dropped. The shoe engages the road surface and the loss of energy caused by the shoe sliding on the road surface measured by the distance the pendulum rises to the end of its swing is a measure of the frictional characteristic of the skid resistance of the road surface. The Lynch device is a modification thereof since it utilizes a wheel and pendulum arrangement.
Finally, there is the Penn State Drag Tester. This type of surface friction tester is represented by Kummer U.S. Pat. No. 3,301,039. Kummer states that his device is suitable for use on a slope or a curve or to measure large areas where the laboratory tester (pendulum-type) is unsuitable and the high speed road tester (automobile or trailer mounted wheel-type) cannot be used. The Kummer surface friction tester has a wheeled cart which is pushed in order to drag a shoe over the road surface. A hydraulic chamber, filled with a liquid and communicating with a pressure dial, is used to determine the drag of the shoe, and therefore, to show the skid resistance of the road surface. The hydraulic chamber and its connecting tubing have a volume adjustment according to temperature so that the Kummer surface friction tester can be used under variable conditions of sun, wind and rain.
Still, the hydraulic arrangement is a complicating factor which makes the Kummer device relatively expensive to construct. Besides, as Lynch notes, none of these surface friction testers, including that of Kummer, give entirely satisfactory results under various operating conditions. This is particularly a problem when the coefficient of friction of surfaces (asphalt, concrete, gravel, sand, grass, etc.) must each be measured in order to determine the velocity at the point of brake locking of a wrecked vehicle from the skid marks remaining.
Accordingly, the need exists for an inexpensive, portable surface friction tester which can be used under such conditions to determine the coefficient of friction of various surfaces in making an accident report.